1. Field of the Invention
The present invention generally relates to battery powered electronic devices, and more particular, to electronic devices having primary and auxiliary battery supplies of differing voltages.
2. Description of the Related Art
Battery powered personal computers are becoming very popular, particular as the computers get smaller and smaller. This makes them highly portable so that they can be used in practically any location, often quite far from sources of electrical power. This possible inaccessibility of power means that long term battery operation is very important. When going to a remote location it is common to utilize several sets of additional batteries which have been charged prior to leaving the source of available power. As a battery discharges, it is replaced with a charged battery and operations can continue. In this manner the total operating time of the computer before a power down or loss of power situation can greatly exceed that of a single battery.
However, one problem that can occur in the battery changing situation is that the user may not be able to save the information present in the computer when the battery needs to be replaced and before the battery reaches a dead battery level. That is, the battery may need to be replaced but the computer may be in the middle of an operation which cannot be interrupted. In these cases any data present in the memory of the computer would be lost when the dead battery level is reached and any operations would have to be completely reinitiated and any data reentered. This is obviously an undesirable condition. To this end the computer system may contain a smaller, low power auxiliary battery which is not capable of providing energy to a fully operating unit, but can operate a temporary power source if the computer is in a standby or low power mode. The use of the auxiliary battery allows the primary battery to be replaced Without loss of data to the computer system. The use of the auxiliary battery thus allows for extended operation of the computer system without loss of data.
Typically the auxiliary battery will have a lower output voltage than the primary battery. The auxiliary battery preferably is very small and so fewer individual cells are used to form the auxiliary battery, resulting in this lower voltage. However, the computer system contains battery voltage monitoring circuitry to shut down the power supply, and consequently the computer, to prevent damage to the primary battery by deeply discharging the battery and to the computer should a primary battery having too low of a voltage be installed. The voltage of the auxiliary battery is well into the region that would normally be considered dead or fully discharged if the primary battery was active. Yet the auxiliary battery is fully operational and the computer system can continue to operate in the standby mode. Thus the battery voltage monitoring circuitry in the computer system must be able to determine when the primary battery is going dead or when it has been removed and operation is being continued on the auxiliary battery.